Heat engine with steam supply device

ABSTRACT

A heat engine, in particular an aircraft engine, having a first compressor for supplying a combustion chamber of the heat engine with air and a first turbine arranged downstream of the combustion chamber for driving the first compressor, wherein the heat engine also has at least one steam supply line for supplying steam from a steam source into the combustion chamber. The heat engine also has a steam supply device, which has a second compressor and is designed to compress the working gas further by the second compressor as a function of a mass flow conducted through the steam supply line, before the working gas flows into the combustion chamber.

BACKGROUND OF THE INVENTION

The present invention relates to a heat engine, having a first compressor for supplying at least one combustion chamber of the heat engine with air, a first turbine for driving the first compressor, and a steam supply device for supplying steam into the combustion chamber, and relates to an aircraft, in particular an airplane, having the heat engine, as well as a method for operating the heat engine.

Known from WO 2019/223823 Al is an airplane propulsion system, which represents a combination of a gas turbine cycle process and a steam turbine process in a single machine. In a steam generator arranged downstream of the turbine, steam is produced by means of exhaust-gas energy and then supplied to the machine in the region of the combustion chamber. The higher mass flow in the turbine due to the addition of steam brings about an increase in power output, and, owing to the recovery of heat, the efficiency of the gas turbine is improved. After flowing through the steam generator, the moist gas passes through yet further components, which serve to separate the water from the exhaust gas in order that it becomes available for the steam generation.

With increasing proportion of steam in the working gas of the gas turbine, the specific power output thereof and also the thermal efficiency thereof increase. In particular, when a gas turbine with steam injection (as in WO 2019/223823 Al) is provided for airplane propulsion, the aim is therefore to achieve a high proportion of steam in the working gas. It is possible in this way to achieve a very high specific power output, as a result of which the size and the weight of the components through which the working gas flows can be kept small.

At the same time, it should be possible to achieve a stable operation with little steam and even without any steam whatsoever during, for example, startup or shutdown of the machine in the case of very low partial load or in the event of malfunction when the steam system does not supply any steam. It should thus be possible to operate the machine both as solely a gas turbine and as a combination of a gas turbine and a steam turbine.

In important operating points (takeoff, climb, cruise), for example, it is possible to strive for a supply of approximately 30 mass% steam. If one ignores the air mass flow diverted for purposes of cooling the compressor and if one ignores the fuel mass flow supplied into the combustion chamber, then, in this case, the mass flow of the turbine is approximately 30% larger than that of the compressor. For operating points without steam, the mass flow of the turbine is similar to that of the compressor. The turbine mass flow thus varies substantially because of the supply of steam.

The narrowest cross section in the flow channel of an engine downstream of the combustion chamber is usually the guide vane cascade in front of the high-pressure turbine (directly following the combustion chamber). With increasing pressure drop over the turbine cascade, the outflow Mach number thereof increases and ultimately reaches the supersonic range. In this state, the speed of sound is attained in the narrowest cross section of the blading and the cascade starts to undergo stalling. This means that the maximum reduced mass flow rate thereof is reached:

${\overset{˙}{m}}_{red,\max} = \overset{˙}{m}*\frac{\sqrt{T}}{p}$

The reduced mass flow that is reached in the stalled state is referred to as the capacity and is directly related to the narrowest flow cross section. Accordingly, the capacity is a fixed value that belongs to a turbine and does not change between the operating points. In a conventional engine, a critical flow occurs over nearly the entire operating range of the high-pressure turbine. Thus, for a given total pressure p and a total temperature T, the natural mass flow rate ṁ no longer changes. Therefore, the turbine determines the mass flow rate.

As described further above, it is possible in a propulsion concept in accordance with the invention to introduce, for example, up to 30% additional steam mass flow into the region of the combustion chamber. The result thereof would be that the working point or the working line in the characteristic diagram of the compressor lying upstream would be subjected to strong fluctuations. In the case of large quantities of steam, the compressor would be strongly throttled, thereby giving rise to the danger that an unstable state, so-called “pumping,” ensues. Conversely, if no steam were supplied, the compressor would be dethrottled, as a result of which the working point in the characteristic diagram would become lower. In this operating range, the pressure buildup would then be lower and the efficiency would be markedly poorer.

A design of the compressor for these conditions in accordance with conventional methods (for example, by use of variable guide vanes in the compressor) is difficult and entails many drawbacks. A great deal of variable geometry would be necessitated and, despite this, there would still exist the difficulties of operating near the pumping limit or of operating when there is low pressure buildup and poor compressor efficiency.

A further possibility of keeping constant the position of the working line in the compressor characteristic diagram could be the introduction of a turbine with variable capacity. In this case, a drawback would be the high complexity and the strongly negative influence on turbine efficiency.

SUMMARY OF THE INVENTION

Against this background, the object of the invention is to improve a heat engine having steam supply or to improve the operation thereof.

This object is achieved by a heat engine of the present invention. An aircraft with a heat engine is described below as well as a method for operating a heat engine. Advantageous embodiments of the invention are discussed in detail below.

In accordance with an embodiment of the present invention, a heat engine, in particular an aircraft engine, has at least one first compressor for supplying a combustion chamber of the heat engine with air and a first turbine or high-pressure turbine arranged downstream of the combustion chamber for driving the first compressor. In other words, the heat engine has the typical main components of a gas turbine, namely, a gas expansion turbine, a preceding first compressor, and an intervening combustion chamber, in which a fuel (for example, kerosene or hydrogen) undergoes combustion. In accordance with the present invention, the heat engine has, in addition, at least one steam supply line for supplying steam from a steam source into the combustion chamber as well as a steam supply device. The steam supply device comprises a second compressor, which is arranged downstream of the first compressor and is designed to compress further the air conveyed by the first compressor at least temporarily as a function of a steam mass flow conducted through the steam supply line, before the air enters into the combustion chamber. In particular, the second compressor can be arranged preceding the combustion chamber as the last-most downstream compressor stage(s). Preferably, the steam supply line can be designed for the purpose of operating the second compressor in such a way that, when there is an increase in the steam supply into the combustion chamber, the second compressor brings about a higher compression of the air and vice versa. Further preferably, the steam supply line can be designed so as not to operate the second compressor or to shut it down when no steam is supplied into the combustion chamber.

Preferably, the steam supply device can be designed to drive the second compressor in such a way that the throttled state of the first compressor does not change when steam is being supplied; that is, the stationary working line in the compressor characteristic diagram remains unchanged. The position of the working line is accordingly independent of the supplied quantity of steam. That is, when there is a change in the mass flow at the outlet of the combustion chamber on account of a supply of steam, an increase in the pressure of the air that is proportional to said change can occur by way of the second compressor. Thus, for example, when no steam is supplied, no additional compression occurs by way of the second compressor. In the case of a 10% steam supply into the combustion chamber (that is, in the case of a mass flow ratio

$\frac{\overset{˙}{m}\mspace{6mu} after\mspace{6mu} combustion\mspace{6mu} chamber}{\overset{˙}{m}\mspace{6mu} before\mspace{6mu} combustion\mspace{6mu} chamber} \approx (1.1),$

the second compressor can be operated correspondingly in such a way that it achieves a pressure ratio of 1.1; for a steam supply of 30%, the second compressor can be operated in such a way that it achieves a pressure ratio of 1.3, etc.

In other words, in accordance with an embodiment of the present invention, it is possible to operate the engine in various operating states with different steam proportions in the mass flow from the combustion chamber onwards. In this case, the steam supply device can be designed and set up in such a way that it drives the second compressor in such a way that the working line or the respective working point in the first compressor between two different operating states (for example, 0% steam proportion and 20% steam proportion) deviates by a maximum of 10%, in particular by at most by 5%, in one embodiment by at most by 1%, of the assigned pressure ratio, and, in one embodiment, it is identical.

The present invention is based on the following idea: by way of the (higher) steam supply in a first operating state in comparison to a second operating state, the actual mass flow into the combustion chamber and at the outlet of the combustion chamber or at the inlet thereof increases in at least one turbine. By way of a corresponding design or a corresponding operation of the steam supply device and, in particular, of the second compressor, it is possible to compensate for this higher mass flow at least in part through design or operation or it is possible to design the heat engine for both operating states at the same time; that is, it is possible to operate in both operating states equally well, in particular with advantageous efficiencies and reduced risk of compressor pumping.

Due to the fact that the second compressor is not driven by a turbine that is situated downstream of the combustion chamber in the flow channel, it is possible to decouple the pressure increase thereof from the engine core mass flow and to adapt the degree of compression flexibly to the steam proportion and external constraints.

In accordance with a preferred embodiment of the invention, the steam supply device can further have a steam turbine for driving the second compressor. Preferably, at least a part of the steam from the steam supply line is conducted through the steam turbine and undergoes pressure release in it before it flows into the combustion chamber. The second compressor and the steam turbine can be seated here preferably on a common shaft. However, it is equally possible to supply the power output of the steam turbine to the second compressor in another way, such as, for example, via a transmission.

Driving the second compressor by way of the steam turbine or the drive power thereof results in a self-regulating system, because the turbine is driven more strongly with a higher steam mass flow, which, in turn, brings about a higher compression and, accordingly, the throttled state of the upstream first compressor is balanced.

In accordance with a further exemplary embodiment of the invention, the steam source can comprise an evaporator or a heat exchanger, which is designed to evaporate water by using the exhaust-gas heat of the heat engine. Corresponding systems are known, for example, from WO2019/223823 A or WO2020187345 A1, which are incorporated herewith by way of reference in the description. In brief summary, it is possible by mean of a condenser to recover water from the exhaust-gas jet of the heat engine and then to pump it into an evaporator, where the water is evaporated or supercritically heated in order to be supplied into the combustion chamber (and the steam turbine).

In accordance with a further aspect of the invention, a part of the steam from the steam supply line is conducted so as to flow through the steam turbine and another part of the steam is conducted so as to bypass the steam turbine via a bypass line and to flow into the combustion chamber. The power that is recovered from the exhaust-gas heat during the evaporation can markedly exceed the requirement of the second compressor. By diverting a part of the flow of steam for the steam turbine (while the remaining steam is conducted so as to bypass the turbine and enter the combustion chamber), it is possible in a corresponding design to manage the operation of the steam supply device so that the pressure increase at the second compressor occurs in proportion to the increase in the mass flow due to the steam supply.

In accordance with a further aspect of the invention, it is possible by way of a steam control valve to regulate the steam supply in the steam turbine. This allows an even more flexible adaptation of the power output of the turbine. Desirable under some circumstances is an operating mode in which the pressure change temporarily behaves in a nonproportional manner with respect to the change in mass flow due to the steam supply. When, for example, the heat engine is operated with an increased temperature at the outlet of the combustion chamber during acceleration, it may be of advantage to increase the compressor capacity of the second compressor or to increase the pressure ratio thereof in order to lower the working line in the first compressor and thereby to prevent the dangerous compressor pumping in a secure manner. Preferably, the steam control valve regulates a mass flow ratio between the steam turbine and the bypass line.

In accordance with a further preferred embodiment of the invention, the second compressor and/or the steam turbine can be arranged to be non-coaxial with respect to the first compressor. Preferably, the compressor and the steam turbine can each run on their own shaft, which is offset in arrangement relative to the shaft of the first compressor. An offset arrangement has the advantage that the compressor can be designed with a comparatively small hub ratio and thereby with large vane heights. In addition, the turbomachine part of the aircraft engine can be constructed to be axially shorter.

In accordance with a further aspect of the invention, the heat engine has at least one third compressor for supplying the combustion chamber and/or a fan (propulsor) and/or at least one further turbine, in particular for driving the third compressor and/or fan. Further preferably, the fan can be connected to the shaft of the additional turbine by way of a (reduction) transmission. In other words, the heat engine described above can form the core engine of an aircraft engine and can be supplemented by a fan, a low-pressure turbine, and, if need be, a further compressor module.

In accordance with another preferred exemplary embodiment of the invention, the heat engine can have a second steam turbine, through which steam of the supply line likewise flows. The power output of the second steam turbine can be supplied, for example, to a shaft of the heat engine or to an auxiliary unit. The background hereof is that the power recovered from the exhaust-gas heat during the evaporation can exceed the requirement of the second compressor. In order to be able to utilize this excess power, it can be utilized in a second (third, fourth,..) steam turbine for other purposes.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURES

Other advantageous further developments of the present invention ensue from the dependent claims and from the following description of preferred embodiments. Shown to this end in a partially schematic manner are:

FIG. 1 a heat engine in accordance with an embodiment of the present invention;

FIG. 2 a heat engine in accordance with another embodiment of the present invention.

DESCRIPTION OF THE INVENTION

FIG. 1 shows schematically a heat engine 1, which is based on the fundamental principle of the invention. The problem of the large working point shift or working line shift in the compressor 10 during the supply of steam from a steam source 25 to the combustion chamber 11 is solved by the heat engine 1 with a steam supply device 2. The steam supply device 2 has, as its main components, a second compressor 20, which is driven by a steam turbine 21.

Air that is conveyed from the compressor 10 of the heat engine 1 is conducted to the compressor 20 of the steam supply device 2. The steam turbine 21 feeds its power output to the compressor 20, which is designed in such a way that the pressure increase is proportional to the change in mass flow due to the steam. Accordingly present is a self-regulating mechanism, which ensures that the position of the working line in the compressors lying upstream does not change or does not appreciably change when steam is supplied into the combustion chamber 11. That is, in the absence of a steam supply to the steam turbine, the compressor 20 is not driven and does not bring about any pressure increase. For a steam supply of 30% of the air mass flow, for example, there is so much power output that the compressor achieves a pressure ratio of Pi = 1.3. In other words, the steam supply device 2 is designed in such a way that, when the mass flow in the turbine is 30% higher than the mass flow in the compressor, the second compressor 20 also increases the pressure in front of the combustion chamber by 30%.

In the example illustrated, it is thereby taken into account that, in the case of steam-free operation, air conveyed through the first compressor 10 flows through the compressor 20. This causes pressure losses, which are not relevant, however, because what is involved in the case of these operating points is a partial load point.

By use of the exhaust-gas energy of the heat engine 1 in the present exemplary embodiment, water that is pumped from a feed water pump 26 to the steam source 25, namely, in this case, to a steam generator (an evaporator/heat exchanger), is evaporated and subsequently conducted through the steam line 24 to the steam turbine 21. In this case, the steam is brought to a pressure that is very much higher than the pressure in the combustion chamber 11. In this way, the utilizable thermal gradient is increased. After the expansion of the steam in the steam turbine 21, the pressure in the exhaust steam line 23 has to be greater than or at least equal to the pressure in the combustion chamber 11.

The energy flow in the steam supply line 24 is greater than the power required for driving the compressor 20. For this reason, the steam turbine 21 is supplied only with enough energy so that the power is sufficient in order to achieve the desired pressure increase in the compressor 20. The remaining steam is conducted via a bypass line 22 directly to the combustion chamber 11.

The air from the compressor 20 and the exhaust steam from the steam turbine 21 can be supplied directly to the combustion chamber 11. It is also possible for both streams to be mixed beforehand in full or in part. In the combustion chamber, the incoming air and the steam are mixed with fuel (K). The energy-rich working gas resulting from the combustion undergoes pressure release in the turbines 12 and 13.

FIG. 2 shows an exemplary aircraft engine 3 having the heat engine 1 with the supply device 2 shown in FIG. 1 as well as some reasonable augmentations. It could also be stated that the heat engine 1 in FIG. 1 forms the core engine of the aircraft engine 3 (with some minor adaptations). Functionally identical elements have the same reference numbers as in FIG. 1 .

In addition, there is a propulsor, namely, a fan 30, in the example shown, which is driven by a low-pressure turbine 13 and, optionally, is driven via an intervening transmission 31. It is likewise possible to arrange an additional compressor (low-pressure compressor or booster) 32 in the flow direction between the fan 30 and the first compressor 10.

In FIG. 1 and FIG. 2 , the steam supply device 2 is arranged adjacent to or offset with respect to the main axis of the heat engine 1 or of the aircraft engine 3. It is thereby fundamentally irrelevant whether the arrangement is chosen to be parallel, at an angle, or transverse. In an alternative exemplary embodiment, which is not shown here, it would also be conceivable to have a coaxial arrangement. Because an engine with steam in the working gas achieves a very high specific power output, there results a reduced mass flow for the compressor. In the case of a coaxial arrangement of the steam supply device 2, this would result in very small vane heights in the compressor 20. High gap losses and the danger of pumping would be the consequence hereof. The offset arrangement has the great advantage that the compressor 20 can be designed with a small hub ratio and thereby with large vane heights. In addition, the turbomachine part of the aircraft engines can be constructed to be axially short.

As described further above, the energy flow in the steam supply line 24 is greater than the power required for driving the compressor 20. For this reason, it can be advantageous to reduce the excess energy in an (optional) second steam turbine 28. Illustrated schematically in FIG. 2 is a possible embodiment. Here, the second steam turbine 28 is arranged coaxially with respect to the engine axis and feeds its power via an optional transmission 27 to a shaft of the aircraft engine. In the illustration, the steam turbine 28 is connected in series to the steam turbine 21; that is, the steam from the steam source 25 (here the steam generator) first flows through the second steam turbine 28 and afterwards flows through the first steam turbine 21. Also conceivable is a parallel connection of the steam turbines 21, 28 (not depicted). Instead of feeding the power to an engine shaft, the steam turbine 28 could also be used to drive auxiliary units or, for example, a generator.

On the basis of the heat engine in FIG. 1 , it is shown how, by way of the steam supply device, the position of the working line or the working point in the compressor characteristic diagram can be kept constant. In the case of a corresponding design, the steam supply device 2 can also be utilized, however, in order to influence the position of the working point in a specific manner. To this end, a steam control valve 29 is integrated in the steam supply line 24. This steam control valve 29 can be used to vary the quantity of steam of the steam turbine 21. In the case of an unchanged steam mass flow to the combustion chamber, it is accordingly possible to regulate the pressure buildup in the compressor 20. The larger the bypass quantity is, the smaller is the pressure buildup in the compressor 20. As a result, the compressor 10 is throttled and the working point thereof moves in the direction of the pumping limit. In the case of smaller bypass quantities, exactly the opposite behavior is observed.

This can be a great operational advantage. For example, when the engine is accelerated, it is possible to lower the working line in the compressor characteristic diagram and thus to prevent “pumping.” In this way, the engine concept in accordance with the invention makes possible a very large working range, because, besides the turbine inlet temperature, also the mass flow rate can be utilized for adjustment of the power output. By way of influencing the working point in the compressor characteristic diagram, it is possible to improve the dynamic behavior. 

What is claimed is:
 1. A heat engine having a first compressor for supplying a combustion chamber of the heat engine with air and a first turbine arranged downstream of the combustion chamber for driving the first compressor, wherein the heat engine further has at least one steam supply line for supplying steam from a steam source into the combustion chamber, wherein a steam supply device, which has a second compressor arranged downstream of the first compressor and which is configured and arranged to operate the second compressor in such a way that it further compresses the air at least temporarily, as a function of a mass flow conducted through the steam supply line, before it flows into the combustion chamber.
 2. The heat engine according to claim 1, wherein the steam supply device is configured and arranged in at least a first operating mode to operate the second compressor so that the pressure increase of the air is proportional to the change in mass flow due to the steam fed into the combustion chamber.
 3. The heat engine according to claim 1, wherein the steam supply device is configured and arranged at least in a second operating mode to operate the second compressor so that the pressure increase of the air is not proportional to the change in mass flow due to the steam fed into the combustion chamber; wherein the pressure is more strongly increased than an increase in the mass flow.
 4. The heat engine according to claim 1, wherein the steam supply device further has a steam turbine for driving the second compressor and at least a part of the steam from the steam supply line is conducted through the steam turbine and undergoes pressure release in the latter, before it flows into the combustion chamber.
 5. The heat engine according to claim 1, wherein the steam source comprises an evaporator or a heat exchanger, which is configured and arranged to evaporate water and/or to heat it supercritically with the exhaust-gas heat of the heat engine.
 6. The heat engine according to claim 5, further comprising a water recovery unit with at least one second heat exchanger for the recovery of condensed water from the exhaust gas of the heat engine.
 7. The heat engine according to claim 1, wherein a part of the steam from the steam supply line flows through the steam turbine and another part of the steam is conducted via a bypass line past the steam turbine into the combustion chamber.
 8. The heat engine according to claim 1, further comprising a steam control valve, which regulates the steam supply into the steam turbine and controls a mass flow ratio between the steam turbine and the bypass line.
 9. The heat engine according to claim 1, wherein the second compressor and/or the steam turbine are not arranged coaxially with respect to the first compressor.
 10. The heat engine according to claim 1, further comprising at least one third compressor and/or by a fan for supplying the combustion chamber and/or by at least one further turbine for driving the additional compressor and/or the fan via a transmission.
 11. The heat engine according to claim 4, further comprising a second steam turbine, through which steam from the steam supply line flows and feeds the power to a shaft of the heat engine or to an auxiliary unit.
 12. An aircraft, including at least one heat engine according to claim
 1. 13. A method for operating a heat engine according to claim 1, wherein, at least temporarily, steam is conducted via the steam supply line into the combustion chamber and the second compressor is operated as a function of the supplied steam mass flow in order to further compress the air. 